a) Field of the Invention
The invention is directed to a method for contactless determination of product characteristics, particularly in continuous or discontinuous fabrication of layer systems formed of a plurality of layers with different optical characteristics. The invention is further directed to an arrangement for implementing the method. The method and arrangement can be used in a particularly advantageous manner in connection with control and quality monitoring in the manufacture of the layer systems mentioned above.
b) Description of Related Art and Discovery of Problems Leading to the Present Invention
Measuring devices which operate by spectroscopy and by means of which the reflection and transmission of surface portions on products can be detected are known. Optical characteristics of the products and other quality parameters such as dimensional accuracy of selected product zones can be determined from the measurement spectrum.
For this purpose, an optical measuring head is arranged in a known manner in the immediate vicinity of a product to be characterized. Measurement light is directed from a light source in the measuring head onto a selected measurement surface on the product and the light which is reflected, emitted or transmitted by the measurement surface is then fed to a receiver and subsequently evaluated.
Particularly with respect to highly automated production of multilayer systems, it is desirable to measure the layer construction during the arrangement of the layers or, at the least, shortly before each layer is applied in order to obtain current information about the success of the respective manufacturing step or about the quality of the product and so that corrective measures can be initiated immediately in case of deviations.
In order to achieve fast, reproducible results which are capable of delivering factual information, analytic methods are used which are distinguished by a short analysis period, high analysis frequency, high informational content per time unit, robustness for process installation and operation, reliability, automatability, small space requirement, versatility of detection systems.
However, in the production of layer systems such as have been developed in particular for solar technology, the optical measurement methods and arrangements available in the art are not suitable for carrying out reflection measurements and transmission measurements constantly and with sufficient speed over the entire width of an optical spectrum. Measurements for this purpose are currently still very time-consuming and cost-intensive which, above all, slows down the pace of the manufacturing process.
For this reason, the requirements demanded for production quality have increased considerably in recent years. In particular, requirements for consistency of composition, morphology and, above all, layer thickness are high. In multilayer systems, for example, in the semiconductor industry, in the industrial manufacture of anodizing layers or in the production of modern solar cells based on cadmium sulfide, tight tolerances must be maintained during the course of production.
For purposes of quality control, offline processes are still often used in the art, i.e., spot samples are taken and measured in the laboratory close to operations. However, a direct online or inline measurement during running production operation with, as far as possible, simultaneous detection of different current quality parameters would be desirable.
Optical and spectroscopic methods detect the chemical composition through the interaction of light with the surface by selective absorption and also detect morphological variations through the measurement of scattered light. When the phase relationships between waves are also observed through a suitable arrangement, facts about the layer thickness, for instance, can also be deduced from the interference. In principle, optical and spectroscopic methods are particularly suitable as online and inline measurement techniques because they work very quickly and without contact.
The measurement of interference phenomena is established in the art. Ellipsometric systems have been developed which can determine up to five layers simultaneously. However, for this purpose, all layers must be transparent. In addition, the use of this technique as an online method causes considerable difficulties because it functions reliably only with smooth surfaces. Determination of layer thickness by means of measuring spectral interference is still not considered usable online and its use is restricted to layer thicknesses preferably of 1 μm or more.
With spectral interference, the layers are typically measured in the wavelength range of visible light from 400 nm to over 700 nm. Usually, a transparent layer (e.g., oxide layer) is applied to an opaque layer (e.g., metal). Accordingly, due to the surrounding light, a three-layer system is formed (with three different refractive indices) which can be described very easily by applying the Fresnel formula. Measurements can be carried out in specular arrangements as well as in diffuse arrangements. The layer thickness is calculated by the interference or specific absorption.